Archaeological Conservation

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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.

THIS ENTRY IS A DRAFT

Archaeological conservation is a profession devoted to the preservation of the archaeological record including large-scale features such as sites, structures, and landscapes, as well as artifacts. Archaeological conservation is guided by ethical principles that derive from the understanding that these materials are "primary resources for understanding and interpreting the past."[1]

Archaeological remains are found throughout the world in areas of past and current human habitation. Archaeological explorations uncover artifacts found in wide-ranging places, from objects buried deep under the sea [2] and in the most extreme environments that humans have explored[3] to materials found practically in our own backyards. Consequently archaeological conservators work all over the globe: in the field as well as in museums, collections and in private practice.

While excavation allows us to learn a great deal about the past through studying archaeological remains and their context, exposing artifacts that have been buried can cause rapid and irreversible deterioration. Archaeological conservators work to stabilize and preserve the material remains of the past, as well as to investigate how things were made, used, and disposed of. Working with other archaeological professions (e.g. bio-archaeologists, paleobotanists, architects, archaeochemists, art historians, etc.), conservators contribute to a better understanding of the past.

Archaeological materials and context

Archaeological conservators can work on a large scale (on sites, architectural structures, and landscape features) as well as on a smaller scale on movable artifacts. In a broader context, conservators are also sometimes called upon to help preserve intangible aspects of cultural heritage, for example sightlines or the historic roads and paths which structure visitor experience. Though almost all archaeological conservation thus far has occurred on Earth, there is concern about the future preservation of historic information like the footprints left on the moon by the Apollo astronauts. [4]

In situ remains vs. movable objects

At many archaeological sites complexes of buildings or monuments are found in relation to one another and remain in situ while smaller objects may be removed from the site for further study and safe storage in secure depots. Large building structures, floor mosaics, walls and wall paintings, altars, and other large monuments are often left in situ and therefore have different preservation needs from those smaller objects which have been removed from the site. In addition to exposure to the elements, archaeological remains on sites are exposed to visitor traffic, vandalism, and iconoclasm.

Conservators working in the trench to stabilize finds. Courtesy Archaeological Exploration of Sardis (Harvard University)

The movement (and subsequent treatment) of archaeological materials can be a highly contentious subject, and archaeologists as well as conservators should be aware of the deep literature on the political, historical and sociological ramifications of archaeological practice. Today, most archaeological materials remain in the countries where they are excavated, in local storage depots or local or national museums. In the past, foreign excavations made legal arrangements with the government to share and bring back some of the archaeological finds, so many archaeological materials were removed from sites and brought to museums or university collections in other countries. In addition, the legal and illegal trade in antiquities has brought many objects far from their original contexts. Archaeological conservators are bound by professional ethics to preserve archaeological remains and evidence of their context, but individual cases may raise additional ethical questions.

Deterioration and preservation

Burial environment affects the deterioration and preservation of archaeological remains. Exposure to moisture, oxygen, heat, and agents of biodeterioration speed up decomposition in burial. For terrestrial environments, soil conditions (e.g. moisture, pH, salinity, and ground water penetration) have a large effect on the types of materials that are preserved.Inorganic materials tend to remain better preserved in a wider variety of conditions compared to organic materials. Of course, inorganic materials also change through burial: metals corrode and weaken, and porous materials like stone and ceramic can be broken or absorb harmful salts. For the most part,organic materials do not survive well in burial conditions that subject them to periodic moisture, allowing organic decomposition and biological attack. Organic materials are usually best preserved in hot, dry environments (like in Egypt) or in wetter, colder environments where they remain in relatively constant temperatures with low oxygen exposure. Surprisingly, sometimes organic materials do survive in marine environments (e.g. wooden shipwrecks). Waterlogged materials require special care after excavation and organic materials are often consolidated or impregnated with conservation materials (e.g. Polyethylene glycol [PEG] or other resins, or polysaccharides) to allow their study after excavation from marine environments.

Finding an archaeological conservator

This section should include the basic info about who archaeological conservators are and how to find one that is best for a particular project. (sugg. 3 paragraphs)

AIC Find a Conservator tool

How to ensure a good fit for your project

(Types of information that an archaeologist should provide about a site ex. types of artifacts expected, timing, where artifacts will be stored, equipment on site, etc., and questions to ask an archaeological conservator)

Theoretical approaches and ethical considerations

Include general statements touching on each of these topics, roughly 1-2 paragraphs for each bullet points and referencing significant publications on these topics.
a. Objectives of archaeological conservation (data preservation; facilitating interpretation of a site/object; preserving structural, chemical, and physical integrity of materials for future study and use; accessibility)
b. Processes of conservation: investigation, intervention, preventive care/maintenance
c. Collaboration and communication (with archaeologists, research specialists, scientists; national/regional antiquities authorities; local community; practical adaptations to limited economic/human resources)
d. Objects with unknown provenance, illegally exported material, looted artifacts
e. Standards for practice and codes of ethics (bullet point list of conservation and archaeological organizations, museums with hyperlinks)
f. US federal legislation; international treaties, conventions, and charters for the protection of cultural property (annotated bullet point list with hyperlinks where possible)

Links to codes of ethics

Archaeological conservation in practice

An introduction to where archaeological materials may be encountered in conservation, distinctions/generalizations of working with archaeological materials in these contexts, and references to major publications. (sugg. paragraph for each subheading)

Archives and repositories

Laboratory conservation

(institutional and private collections)

Community outreach

Conservation and care of specific archaeological materials

Siliceous materials

Metals

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.
To find a conservator, please visit AIC's Find a Conservator page. For more information visit AIC's Resource Center.

The treatment of freshly excavated archaeological metals is a complicated subject. Metal preservation varies based on types of objects, soil composition and pH, and exposure to the elements, all of which are variables specific to each archaeological site. Even within a single site, there can be significant variation. Conservators working on site may face limited time, space, and other resources, particularly with regard to analytical equipment and chemicals normally available in more established laboratories. Treatment of metal objects on an archaeological site is typically broken down into two phases: on site (in the trench) stabilization for excavation and lifting (involving conservators as necessary) and cleaning, stabilization, and repair in the lab after excavation.

Stabilization for Excavation and Lifting

Conservators working in the trench to help excavate and stabilize finds before lifting. Courtesy Archaeological Exploration of Sardis (Harvard University)

Archaeological metals are likely be structurally deteriorated due to corrosion in the burial environment. Metal tools should be avoided, and softer tools like wooden or plastic skewers, spatulas, brushes, or air puffers should be used to loosen the soil around the object to prepare for lifting without damaging the surface. Facing with tissue may be necessary to preserve the arrangement of dislocated fragments in order to safely remove the metal artifacts from the ground. If metals are moist while in the ground, they should be kept damp to avoid rapid drying until they can be treated by conservators. Metals should be cushioned during transport to the laboratory. An initial identification from the excavator will help determine the course of treatment in the conservation lab, but burial dirt can obscure important details, such as remnants of gilding or surface inscriptions, so metals should be examined by a conservator as soon as possible after they are removed from the ground. In complicated situations, additional measures such as applying a layer of cyclododecane or additional support from aluminum foil can be used to allow fragile objects to be lifted from the ground.

Cleaning, Stabilization, and Repair

Conservator working in the lab to clean finds after excavation. Courtesy Archaeological Exploration of Sardis (Harvard University)

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Generally, conservators try to use the least interventive approach possible with the goal of stabilizing the metal object to enable study, safe handling, and long- term preservation in proper storage conditions. For metals such as lead, gold, and silver, this may indeed mean “minimal” treatment when the objects are well preserved. These metals are frequently only lightly brushed off and are generally cleaned without the use of water or solvents. They may occasionally be coated, but usually only when intensive handling is expected.

Conservator working in the trench to stabilize an iron chair before lifting. Courtesy Archaeological Exploration of Sardis (Harvard University

Archaeological iron may also vary in condition, with surfaces obscured by iron oxide corrosion products, while some artifacts are completely converted to magnetite from contact with the burial environment. Typically iron objects are cleaned of surface dirt with soft tools and may be coated/consolidated if in fragile condition due to extensive corrosion and/or mineralization. Coating is usually carried out after a soak in solvent to dehydrate the metal. In some cases, iron objects may continue to “sweat” or “weep” due to continued corrosion from exposure to high RH environments after excavation. (See CCI Notes 9/1 and 9/6 for more information).

Copper alloys (bronze, brass, arsenical copper alloys, etc.) vary widely in their state of preservation upon excavation due to their composition, history of use, and burial conditions. In the field, conservators generally categorize objects as copper alloy (a generic term) unless it is possible to make a more specific identification through microchemical tests or other analysis. While some copper alloys are well preserved, many suffer from what is termed “bronze disease,” and conservators must intervene to prevent further corrosion of the metal post-excavation that can have devastating effects and cause the eventual disintegration of the object. Special care must be taken with gilt or silvered surfaces, the upper layers of which may be compromised due to corrosion of the underlying copper-alloy metals.

“Bronze disease is a progressive deterioration of ancient copper alloys caused by the existence of cuprous chloride (nantokite) in close proximity to whatever metallic surface may remain. Cuprous chloride may lie dormant until reaction with moisture and oxygen causes this unstable compound to expand in volume on conversion to one of the copper trihydroxychlorides. This creates physical stress within the object affected, resulting in cracking or fragmentation.” (Scott 2002, p 125)

While studies of bronze disease date back over 100 years (with early work by Rathgen and Berthelot), the reactions between the copper corrosion products formed in burial are thermodynamically complex and not yet completely understood (Scott 2002, 129). One assumption has been that high (free) chloride contents in excavated metals indicate susceptibility to further corrosion, and that cleaning and desalination are needed to stabilize the metal. Historically many different treatments have been undertaken to preserve copper alloys from archaeological contexts, both in the field and later in museum collections. In recent decades, treatment of archaeological metals on site has typically included one or more of the following steps: (1) mechanical cleaning, usually aided by (2) water or solvents, (3) desalination through soaking in deionized water, (4) treatment with corrosion inhibitors such as benzotriazole, and (5) coating with synthetic resins to provide a barrier layer to protect the object from handling. Other chemical, electrochemical and electrolytic methods have also been used to clean copper alloys, through these techniques can be quite damaging and may aggressively strip away surface patina if not performed in a controlled manner. Conservators should always try to document and preserve surface information embedded in corrosion products, such as pseudomorphs indicating contact with other materials (e.g. textiles or wood).

Chemical structure of Benzotriazole

Corrosion inhibitors

Benzotriazole (BTA) as a corrosion inhibitor has been extensively used but questions remain about its efficacy. It is usually applied in an ethanol solution in low concentrations with the aid of a vacuum dessicator (this protocol seems to have been in use since at least the 1970s at the Institute of Archaeology, London) (Scott 2002, p379). BTA molecules are thought to complex with CuCl preventing further bronze disease outbreaks from occurring. However, in many cases the chloride corrosion is not stabilized by one treatment of BTA, and other methods may be required to stabilize problematic objects. Other corrosion inhibitors such as cysteine and AMT (5-amino-2-mercapto-1,3,4-thiadiazole) have also been tested, but BTA remains the most commonly used (see 2015 Copper Alloy Treatment Survey).

This copper alloy sheet metal fragment was cleaned, treated with corrosion inhibitor, and coated. Bridges of Japanese tissue were made on the reverse to stabilize cracks. Courtesy Archaeological Exploration of Sardis (Harvard University)

Coatings and Adhesives

Many metals found in excavation are fragmented and may need to be reconstructed after cleaning and initial study. Metals are often coated to prevent damage from handling, and the coating additionally consolidates the surface in cases in which corrosion has caused loss of structural or surface integrity. Today, stable, non-yellowing, and reversible acrylic resins such as Paraloid B72, B44 or B48N [Rohm and Haas] are commonly chosen for this purpose, and these same resins are used in thicker concentrations as an adhesive for joining metal fragments.

Preventive measures to prevent further corrosion include storing the metal in a dry storage environment, as exposure to moisture and oxygen lead to active corrosion. Some conservators have experimented with the use of oxygen scavengers and non-permeable films to encapsulate metals for additional protection (e.g. the Revolutionary Protection "RP" System). Proper protective housing using archival quality materials is important to protect the objects from harm while stored in an archaeological depot or museum storage.

Agnew, N. and J. Bridgland, eds. 2006. Of the past, for the future: integrating conservation and archaeology. Proceedings of the Conservation Theme at the 5th World Archaeological Congress, Washington, D.C., 22-26 June 2003. Los Angeles, The Getty Conservation Institute.

Ashurt, J., ed. 2007. Conservation of ruins. Oxford: Elsevier Ltd.

De la Torre, M., ed. 1997. The conservation of archaeological sites in the Mediterranean region: report on an international conference. Los Angeles: The Getty Conservation Institute.

International Council of Monuments and Sites. 1964. The international charter for the conservation and restoration of monuments and sites (The Venice Charter). Available on-line at: http://www.icomos.org/charters/venice_e.pdf

Stanley-Price, N. and R. Burch, eds. 2004. Special issue on site reburial. Conservation and Management of Archaeological Sites. 6(3-4). London: James and James.

Stanley-Price, N. and F. Matero, eds. 2001. Special issue on protective shelters. Conservation and management of archaeological sites. 5(1-2). London: James and James.

Stewart, J. 2012. The stabilization and protection of archaeological sites from natural processes. In Selected readings from a course in the International Centre for the Study of the Preservation and Restoration of Cultural Property ATHAR Programme (Conservation of Cultural Heritage in the Arab Region): Issues in the Conservation and Management of Heritage Sites, ed. International Centre for the Study of the Preservation and Restoration of Cultural Property. Sharjah: International Center for the Study of the Preservation and Restoration of Cultural Property. 83-91.

Keene, S. 1984. The performance of coatings and consolidants used on archaeological iron. Adhesives and Consolidants. IIC Paris 1984. International Institute for Conservation of Historic and Artistic Works.

Knight, B. 1990. A Review of the Corrosion of Iron from Terrestrial Sites and the Problem of Post-Excavation Corrosion. The Conservator (14): 37-43.

Golfomitsou, S., and J. Merkel. 2007. Understanding the efficiency of combined inhibitors for the treatment of corroded copper artefacts. METAL 07 Proceedings of the Interim Meeting of the ICOM-CC Metal Working Group (5): 38-43.

Sharma V.C., U.S. Lal, and M.V. Nair. 1995. Zinc dust treatment-- an effective method for the control of bronze disease on excavated objects. Studies in Conservation (40): 110-119.

Weisser, T. 1987. The Use of Sodium Carbonate as a pretreatment for difficult-to-stabilise bronzes. Recent advances in the conservation and analysis of artifacts. Jubilee conservation conference, London 6-10 July 1987. University of London, Institute of Archaeology, Summer Schools Press. 105-108.

Packing and storage

Braun, T.J. 2007. An alternative technique for applying accession numbers to museum artifacts. Journal of the American Institute for Conservation 46(2): 91-104.

Cleere, H., ed. 1989. Archaeological heritage management in the modern world. In One world archaeology, vol.9. London and Boston: Unwin Hyman.

Kerr, J.S. 1987. The Australian ICOMOS charter for the conservation of places of cultural significance (the Burra Charter); guidelines to the Burra Charter: cultural significance; guidelines to the Burra Charter: conservation policy. Sidney: Australia/International Council on Monuments and Sites.

Sebastian, L. and W.D. Lipe, eds. 2009. Archaeological and cultural resource management: visions for the future. Santa Fe: School for Advanced Research Press.

Toman, Jiri. 2005. The protection of cultural property in the event of armed conflict. Paris: UNESCO.